The goal of this project is to determine the structural and functional properties of protein complexes formed between purified reovirus particles and expressed reovirus proteins. 1. Overexpressed outer-capsid proteins mul and sigma3 bind reovirus cores in a manner that recapitulates outer-capsid assembly in vivo. The consequences of these binding interactions on the structural and conformational properties of the reovirus particle will be ascertained. First, negative-stain EM will be used to assess sample quality, followed by cryo-SEM to obtain high-resolution structural information. 2. Overexpressed reovirus outer-capsid protein sigma3 binds reovirus ISVPs in a fashion that renders these reassembled particles functionally similar to virions. The structurd~of the reassembled particles will be studied by negative-stain EM and cryo-SEM. 3. Overexpressed reovirus nonstructural protein muNS binds reovirus cores. The effects of this binding interaction will be determined by negative-stain EM and cryo-SEM. Studying these three virus-protein complexes by a combination of EM techniques available through the IMR should yield important insights into the assembly, disassembly, and modes of pathogenesis of this enteric virus, whose relatives include the mammalian rotavirus, a major killer of infants in Tbird World countries.